There is a continued need to find improved ways to rapidly detect signals of interest, including signals providing data indicating the presence of human, animal or equipment activity. The earlier the detection and classification of events, the more time there is available to protect human beings and physical assets that may encounter threatening activities, e.g., military operations, border intrusions and trafficking of illegal goods. More timely responses can be had by reducing the time required for detecting and classifying signals of interest with minimum acceptable confidence levels.
Improvements in signal detection require finding practical ways to increase sensitivity of detectors deployed in field monitoring activities, i.e., to discriminate between low energy emissions and background noise. Improvements in classifying signals of interest can also result from improved detector sensitivity because larger amounts of data can be acquired from signals emerging out of background noise sooner. This can expedite meeting minimum acceptable confidence levels. In turn, this enables more rapid discrimination between similar source types. Reductions in system size and cost can enable more economical deployment of a greater number of sensors.
In the past it has been commonplace to employ multi-modal sensing schemes to characterize such activities of potential interest in an automated or quasi-automated manner. For example, it is conventional to combine different types of sensor systems to discriminate between source types. There may be acquisition of temperature information, infrared data and magnetic field data, in combination, to confirm the presence of a specific object such as a type of terrestrial vehicle. Such systems are complex and may be large.
As noted in U.S. Pat. No. 9,057,796 (the '796 patent), multi-modal sensing systems are not well-suited for rapid deployment, in part because they generally consume levels of power that make long term battery operation impractical. Typically, these systems have identified objects of interest on the basis of data matching where the source of a signal of interest, e.g., a moving motor vehicle, is in a class having a specified signature, and an image is acquired to accompany the multi-mode raw or extracted data. It has been a common approach to acquire time varying power density and spectral data associated with specific sources of seismic or acoustic energy and compare this acquired information with a fingerprint template for a specific vehicle type to determine whether the vehicle is, for example, a motor cycle or a large truck. Due to the varied nature of signatures within a category of vehicles (e.g., moving trucks), fingerprint matching techniques may have an unacceptably high rate of false detections, or they may fail to identify a vehicle as being in a suspect class.
Given the need to provide systems and methods which enable rapid detection of specific types of sources with high levels of confidence, the '796 patent teaches a sensor device that, instead of receiving signal information from a transducer element distinct from a piezoelectric cable, couples the piezoelectric cable directly with the seismic wave field. Such a device is referred to herein as a seismic-acoustic Piezoelectric Cable Sensor Device (PCSD). To effect this arrangement, instead of requiring that the cable be firmly mechanically coupled to a rigid transducer element, such as a cylindrically shaped wall, techniques are provided to establish stable positioning of the cable element along a supporting frame. The cable may be configured in accord with any of numerous configurations, including spiral arrangements such as shown in FIG. 3A of the '796 patent. In the past, piezoelectric cable has been suitable for many seismic field applications, e.g., underground positioning, when environmental conditions demand stabilizing the position of the sensor and protecting the sensor from damage. In fact, it was found that piezoelectric cable can be effectively secured to a stabilizing frame, and encapsulating the sensor material within a durable sheath assures a satisfactory level of robustness for reliable field operations. Nonetheless, the arrangement incidentally results in at least some mechanical coupling between the piezoelectric cable element and the frame. Fortunately, with the sensor primarily embodied as a wrapped cable, mechanical coupling of the cable with the frame could be sufficiently reduced that the predominant means for stimulating the cable with seismic energy could be the direct coupling of the cable with the seismic-acoustic wavefield. Mechanical coupling of the piezoelectric cable to the frame has been reduced by minimizing direct contact between the piezoelectric cable and the frame. Nonetheless, it is difficult to completely prevent transfer of all energy between the frame and the cable.